1. Field of the Invention
The invention relates to microthin conductive, radiopaque metal coatings on nonmetal substrates and in particular to thin, flexible fine wire leads used in vivo for implanted monitoring devices and for signal or communication wires subject to bend and flex stress.
2. Description of Background Art
Cardiac pacing is a proven means of maintaining heart function for patients with various heart conditions. Over 650,000 pacemakers are implanted annually in patients worldwide, including over 280,000 in the United States. Over 3.5 million people in the developed world have implanted pacemakers. Another approximately 900,000 have an implantable cardioverter defibrillator (ICD) or cardiac resynchronization (CRT) device. Pacemakers use an average of about 1.4 implanted conductive leads during their service life and ICD and CRT devices use an average of about 2.4 leads.
Lead failure is a serious and in some situations life threatening problem for pacemakers and ICD and CRT devices. The average number of leads used per unit indicates the high incidence of lead failure, which is due to two main failure modes for current leads.
Failure of a lead body may occur due to cracking or breaking of the conductor in the lead. This often arises due to repeated flexing from the beating of the heart and associated muscular movements that stress the pathway from the pacemaker to the heart. This subjects the lead at a series of points along its length to tens of millions of cycles per year over a lead's lifetime. Currently available wire leads have not been durable enough to withstand this rigorous environment and many have experienced failure due to conductor fatigue.
Recently, the U.S. Food and Drug Administration has asked a medical manufacturer of heart devices to initiate studies to determine the unusual frequency of breakage in wire leads connecting the heart to certain brands of defibrillators. Such failures are potentially life threatening because implantable devices such as defibrillators may be crucial in restoring irregular and abnormal heart rhythms. Breakages of wires inside leads in some of the devices have occurred in as high as 19% of 700 devices in recent use. In an effort to detect potential breakages, the FDA is recommending X-ray studies post implant to detect any potential wire insulation problems, which would indicate future wire breakage.
A second cause of lead failure is dislodgment or fouling of the distal end, thereby rendering the lead inoperable. The distal end can become non-functional in two distinct ways. Dislodgment of the end from the muscle may occur. While not common, the heart muscle in constant motion can unseat the tine, causing lost contact with the area that needs the voltage for correct pacing. A more common occurrence is the buildup of scar tissue around the insertion point due to the body's natural foreign body response and the healing response initiated from insertion of the electrode into the heart. Buildup of scar tissue may also occur from abandoned leads that have not been removed. Increased resistance of an active connection causes a larger draw of power from the generator in order to correctly pace the heart. Both failure modes can be fatal.
The generator/control unit that controls pacing is implanted under but near the skin surface. Leads are routed from the generator to the heart probes to provide power for pacing and data from the probes to the generator. Probes are generally routed into the heart through the right, low pressure side of the heart. For access to the left side, lead wires are generally from the right side through the coronary sinus and into veins draining the left side of the heart. This access path has several drawbacks; the placement of the probes is limited to areas covered by veins; leads occlude a significant fraction of the vein cross section; and, the number of probes is limited to one or two.
Ideally a lead should be evenly coated with a conducting surface so that current flows into a device to which it is connected. Unfortunately metal coatings on nonmetal surfaces such as plastics tend to lack adherence, and often do not cover the entire surface because many coating processes are satisfactory only on flat surfaces. Fixturing, the process for coating complex surfaces, is a challenge with round shapes or surfaces that are not electrically conductive. In many applications, bands, stripes or other radiopaque markers are preferable to whole surface coverage.
Application of a seed layer to non-conductive materials is generally a significant problem. This usually requires complex masking along with the use of a different deposition technique such as sputtering or ion beam assisted deposition (IBAD). This is a costly step leading to handling problems, increased cost due to the double processing and often results in poor coating adhesion.
Plastic parts are not easily metal coated because plastic is a nonconductor of electricity. One approach to this problem has been to use molecular plasma deposition of selected metals onto polymer substrates. As described in U.S. Publication 2007/0178222, controlled conditions for ion plasma deposition can provide evenly distributed metal coatings on polymer substrates, which are both adherent and radiopaque.
Metal clad glass optical fiber waveguides have been reported in U.S. Pat. Nos. 4,407,561, 4,418,984 and 5,002,359. The metal claddings are protective jackets and are not described as microthin films strongly adhered to the underlying core of the fiber.
U.S. Pat. No. 6,282,349 describes a metal beam block surrounding a quartz surgical launch fiber to allow crimping of an extension onto the polymer jacket to hold the beam block and fiber in place. The metal block is intended to alleviate the heat generated from high power inputs required for laser pulsing.